AIR TRAFFIC CONTROL or ATC
Note: This document provides a general overview of Air Traffic Control. It is not more than that and it has no authority, nor can there be any rights derived from it. It is pure informative.Introduction
This document will give a general explanation on what air traffic control is all about. It will indicate the different working units and will explain their working methods in brief. Furthermore there will be a brief explanation about the basics of separating aircraft.Although the situation from real life is taken as a reference, this document is solely aimed for providing information and for use in the virtual world of aviation.What is air traffic control?
The official ICAO* definition reads:Air Traffic Control is a service provided for the purpose of
- preventing collisions:
a. between aircraft, and
b. on the manoeuvring area (airfield) between aircraft and obstructions
- expediting and maintaining an orderly flow of air traffic.
If possible air traffic control will safeguard the environmental aspects. * ICAO = International Civil Aviation OrganisationHow does air traffic control work?
To prevent collisions between aircraft there exists many (traffic) rules. The main concern is about separation. That is the minimum distance between the aircraft itself.
Example. Compare it with the traffic on the road. You have to keep your distance and usually you try to manoeuvre around the other traffic.
In the air however, aircraft can move to all sides and are around us at all sides. Below, above, left and right. To prevent collisions we have to keep a safe distance to all aircraft around us at all sides. This “keeping a distance” we call separate.There are two types of separations:
- horizontal separation (left or right, but as well in front and behind us)
- vertical separation (above and below us)
Now to make it really complicated: All combinations are possible of course. Compare it with a big ball. You are in the middle of it and the other aircraft are around you, at all sides. Outside the ball, but to all sides with sufficient distance away.Air traffic control will try to keep that distance big enough not to have collisions. Or, expressed in the more official way: Air traffic control will provide sufficient separation to prevent collisions.How does separation work?
As said before, there are two kinds of separation: horizontal and vertical.
What is horizontal separation?
To all directions in the horizontal plane there has to be maintained a safe but minimum distance of standard 5 nautical miles (about 8 kms, 1 nm = 1.8 km).
What is vertical separation?
Vertical separation is established by maintaining a safe distance between aircraft flying at different altitudes. One above the other. That is the distance in the vertical plane. Above and below any aircraft a safe distance = height of 1000 feet (about 300 meters) is to be maintained. (1 foot= +/- 30 cm)
In practise this means if not the one than the other. As long as it is not 5 nm, it has to be minimum 1000 ft
Flight Level separation
To determine the height of an aircraft, it makes use of an altimeter, based on a barometer. Flying above the so-called Transition Level means that the altimeter is put to the standard pressure setting of 1013,2 HectoPascal or 29,92 inches. In that case one flies at a Flight Level and not an altitude any more. Simply expressed: Flight Level is the altitude in feet at standard pressure setting without the two zero’s at the end.
Each Flight Level is thus separate by 1000 feet. This applies until FL 290 (29.000 feet). There above the separation is standard 2000 feet. Except in the areas where the so-called RVSM (Reduced Vertical Separation Minima) are applied. In the RVSM area separation above FL 290 is again 1000 feet. However, not all countries apply this RVSM yet.
To allocate Flight Levels there is a difference made in global direction of flight. Standard rule is: “Eastbound” traffic follows the odd levels and “Westbound” traffic follows the even levels.
Example:Eastbound track (000º - 179º) -> odd levels: 70, 90, 110, 130, 150, etcWestbound track (180º - 359º) -> even levels: 80, 100, 120, 140, 160, etc.Above FL290: Eastbound: 290, 330, 370, 410; Westbound: 310, 350, 390, 430Note: There are a number of countries that don’t use the east-west division, but use a north-south direction. Example is France.
Application of RVSM:
Let’s turn shortly back to that horizontal separation.Again, think about the traffic on the roads. If I am there with a fast car behind a heavy slow truck, my distance towards him will become smaller and smaller quite rapidly. One can say that the horizontal separation between me and the truck is becoming smaller and smaller.What to do? Well, I will by-pass the truck. On the road that will be done on the left side (mostly). However, in the air, there are two different solutions of “by-passing”:
1. I will by-pass along side. But, the distance to the other one still has to be at least 5 miles. And, I will pass along the RIGHT (!!) sideor2. I will pass above or below him. I will change my altitude. I will fly 1000 feet higher or lower than the other aircraft.
Crossing traffic
It may sound difficult, but it is not. Think about driving on your bicycle. In front of you there is a person crossing the street. Now, if you would not change your course, you may hit this person. So, you decide: Or you deviate a bit to the right and pass behind him, or you deviate a bit to the left and you will pass in front of him. And that is exactly what ATC will do in the air. You will be turned either to the left so as to pass well in front or you will be turned to the left to pass well behind the other traffic.
How does an Air Traffic controller work and what are the means to help him?
The air traffic controller tries to imagine a picture of the air traffic that he has to separate. As if he looks from above, from very high downwards to all the aircraft below him. In reality to do this, he will use a radar system.While overlooking all the traffic on his radar screen, the air traffic controller observes the movements, the changes in distances and eventually the reduction of separation. He will anticipate and react in time to prevent collisions. He will turn aircraft left or right, or he will descent or climb them to provide sufficient separation between them. Horizontally and/or vertically.
How does an Air Traffic Controller know the intentions of a pilot?
First of all, any pilot who will use Air Traffic Services has to announce on forehand what his plans are. Therefore the pilot will fill in a flight plan form. In this flight plan it is indicated, what his intentions are, what his type of aircraft is, where he will start and to where he will fly, which route he will follow, with what speed and how long it will take though. Additionally, because he goes through the air, he will indicate at what flying altitude.All this flight plan information is forwarded to air traffic control and with the use of that information an air traffic controller will be able to make a separation plan. The air traffic controller will know on forehand if the fast car may end up behind a slow heavy truck, as mentioned in the example before.
Note: See further down a special paragraph about flight plans.
How many different air traffic controllers are there?
When we say Air traffic Control most people immediately think about a control tower at an airport, where the air traffic controllers do their work. This is only partly true. In reality there are some more air traffic control units than only that tower.The larger the area of control will be, the more air traffic controllers are needed. That’s why there is a division of airspaces and that’s why there are different air traffic control services and units. Each unit controls its own part and has its own specific tasks.
What are the different air traffic control units?
Basically there are two groups:
- Aerodrome Control These are all the services related directly to the airfield itself.
- Area Control These are the services related to the larger airspace away from the airfields.
Aerodrome Control
Normally exists of:
- Tower control (TWR)
This unit is responsible for
- the movements on the ground
- the traffic during the landing and take-off phase
- the traffic in the immediate vicinity of the airfield (the so-called Control Zone or CTR)
- Approach control (APP): the approach and departure control
This unit is responsible for
- the control of the traffic during the approach phase towards the airfield and the departure phase during its moving away from the airfield after take-off, as well as in a broader range around and away from the airfield in general (the so-called Terminal Control Area (TMA)
Let’s have a closer look at these different units.
Tower control unit, in short TWR
The air traffic controllers in the tower are responsible for all the traffic on the airfield and its direct vicinity. Expressed in a simple way: All the traffic that they can see from their tower view. The area directly surrounding the airfield is called the Control Zone (CTR).They control not only the airborne traffic, but also the traffic on the ground. That is to say, as well aircraft as vehicles or a combination of those. Concerning the vehicles, this is only for those vehicles that are using the areas used by the aircraft, called the manoeuvring area. Thus not the parking or gates.
Example. You may have noticed on airfields that the ground surface has different colours. The colour of the ground surface at the gate may be different from that of the apron itself. That is not only because of the use of different materials, but it indicates the border between the different responsibilities of control. Air traffic control is not controlling at the gates, nor the push-back area. That is the ground-service of the airport authority. Therefore in fact a push-back clearance given by ground-control (GND) is a clearance to enter the by air traffic control controlled taxi-area.The moment, the time of a push-back and/or start-up is set by ATC, because there is a number of events depending on that moment in time. See further.
Since an airfield can be large in scale, it is not always possible to overlook everything from the tower. Therefore use is made of electronic aids, like (ground)radar.In addition, it is not always possible to have only one tower or one tower controller.Therefore sub-divisions are made, dependent on the size of the airfield and the amount of traffic and its complexity. These are:
- Delivery (DEL) that will deliver the flight-plan clearance
- Ground (GND) control, responsible for all (taxiing) traffic on the manoeuvring area, except that at a landing/departure runway(s)
- Tower (TWR) control, responsible for the runway(s), traffic landing and departing and that in the direct vicinity of the airfield.
In times of less traffic, or at smaller airfields, one or more of these tasks could be combined in one position.
Approach (APP) control or the approach and departure service
The air traffic controllers of APP are responsible for the traffic in a wider range around the airfield. We call this area the Terminal Control Area (TMA).Although normally we call it ‘approach’, this service could be split. Especially on the bigger and more busy airfields this will be the case.The commonly used split is between arriving and departing traffic, or ‘arrival’ (ARR) and ‘departure’ (DEP) control. This means that all the departing traffic is controlled by the DEP controller, while the arriving traffic is with ARR.
Area Control service
The area control service (ACC) is responsible for controlling in a large area, not directly related to specific airfields. It is the "remaining" airspace and that above the TMA's around the airfields, the so-called Control Areas (CTA)Area Control could be sub-divided in a LOW and a HIGH part, where inside there maybe several different sectors. An example of Upper Area Control (UAC) is the Eurocontrol Maastricht Upper Control.
Note. Very confusing, but in IVAO the term CTR (center) is used for ACC or Area Control Centre. In real life CTR means the Control Zone around an airfield.
The spatial picture
To make the different area explained more clear, there will be some pictures.
Picture on the right, the small green circle is the ground and the airfield with the tower and the runway. The cylinder on top of this green circle represents the area served by the tower: the Control Zone (CTR). The sight of eye from the tower.
The dimensions of the areas (the cylinders) are mentioned in the relevant publications and often on the navigation charts.
Hereunder left, example from the AIP of the Netherlands of the airfield of Groningen (Eelde) with its TMA (dimensions lateral and vertical from 1500 ft AMSL until FL065) and in the middle the dotted line with the Control Zone, the TWR area (dimensions lateral and vertical from the ground till 3000 ft AMSL).
N.B. Amsterdam FIR means Flight Information Region. That is the whole of the Netherlands airspace (see further).
Now let’s take two airfields and we will put one big cylinder (area) around them. That’s where we put the ‘area control’.The control in between the airfields and the bigger control area (CTA) around them.
Finally, to indicate an even bigger totality, we have added several areas, all connected and all with an approach control and one or more airfields.
Area control 1 Area control 2 Area control 3 Approach 1 Approach 2 Approach 3 Approach 4 Approach 5 Approach 6 Airfield 1 Airfield 2 Airfield 3 Airfield 4 Airfield 5 Airfield 6
Let’s have a look what it means for ATC, if there is a flight from airfield 1 (left) towards airfield 6 (right).From left to right the pilot is subsequently under control of:
DEL 1 GND 1 TWR 1 APP 1 CTR 1 CTR 2 CTR 3 APP 6 TWR 6 GND 6 flightplandelivery taxi Take-off departure climb + overflight overflight overflight+ descent approach landing taxi
All these different services work very closely together. They are link like a chain. That means that they are communicating with each other as well. They co-ordinate with each other and they have standard procedures on the transfer of aircraft from the one controller to the next. It has to be one fluent stream of traffic.THE CHAIN
ATC is like a chain with links. And the strength of this chain is determined by the weakest link.That’s one comparison.There is another one. The stream of traffic is like a funnel. On top you can only throw-in such an amount of liquid as can stream out on the bottom of the funnel. If you throw in too much, you’re shoes may get wet.The combination of these comparisons shows that we have to be careful with overloading the weakest link of the chain. Otherwise the whole system may collapse.In real-life it is know that if there is heavy fog at an important European airfield like London, you may see the effects throughout the rest of the European ATC system. Why?
Aircraft move from one air traffic control unit to the other. There is a constant stream of traffic, from departure until landing. And this stream does not allow for interruptions.In our example here above a flight is on its way from airfield 1 to airfield 6, this aircraft is controlled by the different ATC units. If there is an interruption in one of these services, there will be immediately a traffic jam. If the one unit cannot hand-over its traffic to the next unit, the funnel becomes overloaded and …there will be holdings, effecting up-stream the traffic flow in the previous units.A traffic jam in the air is not always easy to handle. Therefore, keep the aircraft on the ground until the problems in the air are solved. Don’t let them take-off for a while.Still, in reality there are several places where aircraft can hold in the air. The so-called holding-patterns. Normally around airfields, but since the capacity of a holding-pattern is limited as well, there may be en-route holding patterns.Simply said: If the funnel at the airfield is getting full, than en-route has to stop filling the funnel.That is the theory.In real-life it is a bit more complicated. But one thing is the same: There has to be a perfect coordination between all the air traffic units involved. The chain is as strong as its weakest link.Every controller is responsible for his own part of the airspace or airfield. Still he is part of that chain. This means to the up-stream control he has indicated, what he wants and is able to do. And that he has to ask to the down-stream control if he is allowed. Always there are two sides involved.If an Approach controller is starting to drown, he will refuse traffic from Area control. Or he will keep departures waiting.Always try as a controller to avoid an overflow (of the funnel) by timely giving signals into the chain.
A closer look to the flight-plan
Note: There is more information on the flight-plan in this blog at ATC, the chain
Let’s go a few steps backward.The pilot makes a flightplan. In there it is exactly indicated, what he has in mind. The callsign, type of aircraft and its speed, airfield of departure and destination, flightplan routing with the airways in between them and the SID (Standard Instrument Departure) point.A flight-plan is a planning tool that indicates everything that the pilot may know and what he wants to do. But there are three points still missing when the pilot makes his plan:
- the actual squawk code
- the (real) departure time
- the cleared flight-level
These three points are still unknown at the moment the flightplan is being filed.At the moment when these three points become known, they can be put into the flightplan. With that, the flightplan is becoming “active”. One could say, that it is the moment the flightplan will start working. It is like a pre-planned scenario. As from the moment the flightplan becomes “active”, all events thereafter could be calculated. And thus made known.
An example: The flightplan of a Amsterdam departure indicates: EHAM LEKKO B31 WOODY NIK EBBR. This means that the aircraft will follow the standard and cleared departure route for the runway involved towards the point LEKKO (the SID point). Thereafter it will follow airway B31 via WOODY and NIK. Next will be the clearance for a STAR for the active landing runway in Brussels.Now suppose take-off runway in Amsterdam is RWY 24, the flying time via the standard departure route towards LEKKO is 5 minutes, the flying time from LEKKO to WOODY is 6 minutes and from WOODY to NIK is 3 minutes.When we know at what time this aircraft will take off, or has taken off, then we know as well at what time it will enter the Belgium airspace at WOODY. Calculation: T/O time + 5 + 6. That’s 11 minutes after take-off with an average speed of e.g an B737, this aircraft will be under control of a Belgium ATC unit.Why this, maybe, complicated whole story? To be able to control air traffic.Because these calculations can be done with any active flightplan. That means that for every active flight it may be known at what moment what event will take place in time. Or, at what moment an aircraft will arrive where and at which altitude.Remember the story here above about the funnel? With the knowledge of the information of all active flight plans we could make calculations about where there may be an overload in the system. Where holdings may be necessary. Where delays can be expected.In real life, this is being done through so called tactical flow management. Based on actual calculations, traffic predictions could be made.In addition there is long term flow management to provide landing and take-off licences for airfields. One of the reasons here why holiday flight are always very early in the morning, it is their take-off licence. They have to be away before the regular airlines start their flights for the business-men. In other words, spreading the traffic to avoid an overload of the funnel.
The data presentation
With the help of radar we are able to see the air traffic and to project that on a screen. We look at the radar screen and we make an image of the traffic in our minds. Literally, because the radar screen is a flat screen and the imagine has to be spatial, 3 dimensional. In the air space there is one dimension more than on the flat radar plane. The exact position of an aircraft in the air is determined by a direction and a distance from a known point and in addition by the altitude above the ground.
Example: On the radar picture here right we see on the left KLC056. On the right at EHAM there is the indication of the direction to and the distance from the aircraft to EHAM (090/060nm). To make the picture fully spatial there is one item missing: The altitude. And that we can see in the aircraft label on the radar, F176.
These three elements, direction, distance and altitude, together determine the exact position of the aircraft in the airspace.
It means that the air traffic controller has to add in his mind this altitude dimension, when he looks at the radar picture.
A modern radar gives a lot of information and depicts all that on the screen. This is called the digital radar picture. But how does it work?In fact simply by coupling the radar information and the flight-plan information. And hereby the SSR squawk code is the actual coupling factor. A short explanation.
The radar sees the aircraft in the air and shows this as a symbol on the radar screen.The flightplan gives all the information about that aircraft and calculates the events, as from the moment the flightplan has been activated.With the squawk code we tell the system which flightplan has to be coupled to what symbol. The so-called "correlation".In other words: By coupling the flightplan information with the radar information we make the digital radar picture.With that picture the air traffic controller can make an image of the situation in the air and with that image in mind he provides proper separation of the aircraft under his control. He instructs aircraft to fly a specific heading and/or he allocates different altitudes.
That is in short the task and the work of an air traffic controller!
CONCLUSION:As you may have seen the air traffic controller is a busy guy. He keeps an eye on the traffic, he applies different separations and he makes co-ordinations. He applies different traffic rules and he relieves the pilots of some work. He accompanies their flights and he supports them. He provides a service.
Without pilots an air traffic controller cannot do his job. The other way around the work of a pilot is easier and above all more safe with the help of air traffic control.
In other words: It is a working together. Based on rules and agreements. It is a team.
Have a save flight! With ATC :)
Bob (ATCO) van der Flier
© bobatco Utrecht© All rights reserved by Studio Esmeralda doc ATC V2.1 eng - -May 2010
Published earlier and now edited for renewed publication
AIR TRAFFIC CONTROL or ATC
Note: This document provides a general overview of Air Traffic Control. It is not more than that and it has no authority, nor can there be any rights derived from it. It is pure informative.
Introduction
This document will give a general explanation on what air traffic control is all about. It will indicate the different working units and will explain their working methods in brief. Furthermore there will be a brief explanation about the basics of separating aircraft.
Although the situation from real life is taken as a reference, this document is solely aimed for providing information and for use in the virtual world of aviation.
What is air traffic control?
The official ICAO* definition reads:
Air Traffic Control is a service provided for the purpose of
- preventing collisions:
a. between aircraft, and
b. on the manoeuvring area (airfield) between aircraft and obstructions
b. on the manoeuvring area (airfield) between aircraft and obstructions
- expediting and maintaining an orderly flow of air traffic.
If possible air traffic control will safeguard the environmental aspects.
* ICAO = International Civil Aviation Organisation
How does air traffic control work?
To prevent collisions between aircraft there exists many (traffic) rules. The main concern is about separation. That is the minimum distance between the aircraft itself.
Example. Compare it with the traffic on the road. You have to keep your distance and usually you try to manoeuvre around the other traffic.
Example. Compare it with the traffic on the road. You have to keep your distance and usually you try to manoeuvre around the other traffic.
In the air however, aircraft can move to all sides and are around us at all sides. Below, above, left and right. To prevent collisions we have to keep a safe distance to all aircraft around us at all sides. This “keeping a distance” we call separate.
There are two types of separations:
- horizontal separation (left or right, but as well in front and behind us)
- vertical separation (above and below us)
Now to make it really complicated: All combinations are possible of course. Compare it with a big ball. You are in the middle of it and the other aircraft are around you, at all sides. Outside the ball, but to all sides with sufficient distance away.
Air traffic control will try to keep that distance big enough not to have collisions. Or, expressed in the more official way: Air traffic control will provide sufficient separation to prevent collisions.
How does separation work?
As said before, there are two kinds of separation: horizontal and vertical.
What is horizontal separation?
To all directions in the horizontal plane there has to be maintained a safe but minimum distance of standard 5 nautical miles (about 8 kms, 1 nm = 1.8 km).
What is vertical separation?
Vertical separation is established by maintaining a safe distance between aircraft flying at different altitudes. One above the other. That is the distance in the vertical plane. Above and below any aircraft a safe distance = height of 1000 feet (about 300 meters) is to be maintained. (1 foot= +/- 30 cm)
In practise this means if not the one than the other. As long as it is not 5 nm, it has to be minimum 1000 ft
Flight Level separation
To determine the height of an aircraft, it makes use of an altimeter, based on a barometer. Flying above the so-called Transition Level means that the altimeter is put to the standard pressure setting of 1013,2 HectoPascal or 29,92 inches. In that case one flies at a Flight Level and not an altitude any more. Simply expressed: Flight Level is the altitude in feet at standard pressure setting without the two zero’s at the end.
Each Flight Level is thus separate by 1000 feet. This applies until FL 290 (29.000 feet). There above the separation is standard 2000 feet. Except in the areas where the so-called RVSM (Reduced Vertical Separation Minima) are applied. In the RVSM area separation above FL 290 is again 1000 feet. However, not all countries apply this RVSM yet.
To allocate Flight Levels there is a difference made in global direction of flight.
Standard rule is: “Eastbound” traffic follows the odd levels and “Westbound” traffic follows the even levels.
Example:
Eastbound track (000º - 179º) -> odd levels: 70, 90, 110, 130, 150, etc
Westbound track (180º - 359º) -> even levels: 80, 100, 120, 140, 160, etc.
Above FL290: Eastbound: 290, 330, 370, 410; Westbound: 310, 350, 390, 430
Note: There are a number of countries that don’t use the east-west division, but use a north-south direction. Example is France.
Application of RVSM:
Let’s turn shortly back to that horizontal separation.
Again, think about the traffic on the roads. If I am there with a fast car behind a heavy slow truck, my distance towards him will become smaller and smaller quite rapidly. One can say that the horizontal separation between me and the truck is becoming smaller and smaller.
What to do? Well, I will by-pass the truck. On the road that will be done on the left side (mostly). However, in the air, there are two different solutions of “by-passing”:
1. I will by-pass along side. But, the distance to the other one still has to be at least 5 miles. And, I will pass along the RIGHT (!!) side
or
2. I will pass above or below him. I will change my altitude. I will fly 1000 feet higher or lower than the other aircraft.
Crossing traffic
It may sound difficult, but it is not. Think about driving on your bicycle. In front of you there is a person crossing the street. Now, if you would not change your course, you may hit this person. So, you decide: Or you deviate a bit to the right and pass behind him, or you deviate a bit to the left and you will pass in front of him. And that is exactly what ATC will do in the air. You will be turned either to the left so as to pass well in front or you will be turned to the left to pass well behind the other traffic.
How does an Air Traffic controller work and what are the means to help him?
The air traffic controller tries to imagine a picture of the air traffic that he has to separate. As if he looks from above, from very high downwards to all the aircraft below him. In reality to do this, he will use a radar system.
While overlooking all the traffic on his radar screen, the air traffic controller observes the movements, the changes in distances and eventually the reduction of separation. He will anticipate and react in time to prevent collisions. He will turn aircraft left or right, or he will descent or climb them to provide sufficient separation between them. Horizontally and/or vertically.
How does an Air Traffic Controller know the intentions of a pilot?
First of all, any pilot who will use Air Traffic Services has to announce on forehand what his plans are. Therefore the pilot will fill in a flight plan form. In this flight plan it is indicated, what his intentions are, what his type of aircraft is, where he will start and to where he will fly, which route he will follow, with what speed and how long it will take though. Additionally, because he goes through the air, he will indicate at what flying altitude.
All this flight plan information is forwarded to air traffic control and with the use of that information an air traffic controller will be able to make a separation plan. The air traffic controller will know on forehand if the fast car may end up behind a slow heavy truck, as mentioned in the example before.
Note: See further down a special paragraph about flight plans.
How many different air traffic controllers are there?
When we say Air traffic Control most people immediately think about a control tower at an airport, where the air traffic controllers do their work. This is only partly true. In reality there are some more air traffic control units than only that tower.
The larger the area of control will be, the more air traffic controllers are needed. That’s why there is a division of airspaces and that’s why there are different air traffic control services and units. Each unit controls its own part and has its own specific tasks.
What are the different air traffic control units?
Basically there are two groups:
- Aerodrome Control These are all the services related directly to the airfield itself.
- Area Control These are the services related to the larger airspace away from the airfields.
Aerodrome Control
Normally exists of:
- Tower control (TWR)
This unit is responsible for
- the movements on the ground
- the traffic during the landing and take-off phase
- the traffic in the immediate vicinity of the airfield (the so-called Control Zone or CTR)
- Approach control (APP): the approach and departure control
This unit is responsible for
- the control of the traffic during the approach phase towards the airfield and the departure phase during its moving away from the airfield after take-off, as well as in a broader range around and away from the airfield in general (the so-called Terminal Control Area (TMA)
Let’s have a closer look at these different units.
Tower control unit, in short TWR
The air traffic controllers in the tower are responsible for all the traffic on the airfield and its direct vicinity. Expressed in a simple way: All the traffic that they can see from their tower view. The area directly surrounding the airfield is called the Control Zone (CTR).
They control not only the airborne traffic, but also the traffic on the ground. That is to say, as well aircraft as vehicles or a combination of those. Concerning the vehicles, this is only for those vehicles that are using the areas used by the aircraft, called the manoeuvring area. Thus not the parking or gates.
Example. You may have noticed on airfields that the ground surface has different colours. The colour of the ground surface at the gate may be different from that of the apron itself. That is not only because of the use of different materials, but it indicates the border between the different responsibilities of control. Air traffic control is not controlling at the gates, nor the push-back area. That is the ground-service of the airport authority. Therefore in fact a push-back clearance given by ground-control (GND) is a clearance to enter the by air traffic control controlled taxi-area.
The moment, the time of a push-back and/or start-up is set by ATC, because there is a number of events depending on that moment in time. See further.
Since an airfield can be large in scale, it is not always possible to overlook everything from the tower. Therefore use is made of electronic aids, like (ground)radar.
In addition, it is not always possible to have only one tower or one tower controller.
Therefore sub-divisions are made, dependent on the size of the airfield and the amount of traffic and its complexity. These are:
- Delivery (DEL) that will deliver the flight-plan clearance
- Ground (GND) control, responsible for all (taxiing) traffic on the manoeuvring area, except that at a landing/departure runway(s)
- Tower (TWR) control, responsible for the runway(s), traffic landing and departing and that in the direct vicinity of the airfield.
In times of less traffic, or at smaller airfields, one or more of these tasks could be combined in one position.
Approach (APP) control or the approach and departure service
The air traffic controllers of APP are responsible for the traffic in a wider range around the airfield. We call this area the Terminal Control Area (TMA).
Although normally we call it ‘approach’, this service could be split. Especially on the bigger and more busy airfields this will be the case.
The commonly used split is between arriving and departing traffic, or ‘arrival’ (ARR) and ‘departure’ (DEP) control. This means that all the departing traffic is controlled by the DEP controller, while the arriving traffic is with ARR.
Area Control service
The area control service (ACC) is responsible for controlling in a large area, not directly related to specific airfields. It is the "remaining" airspace and that above the TMA's around the airfields, the so-called Control Areas (CTA)
Area Control could be sub-divided in a LOW and a HIGH part, where inside there maybe several different sectors. An example of Upper Area Control (UAC) is the Eurocontrol Maastricht Upper Control.
Note. Very confusing, but in IVAO the term CTR (center) is used for ACC or Area Control Centre. In real life CTR means the Control Zone around an airfield.
The spatial picture
To make the different area explained more clear, there will be some pictures.Picture on the right, the small green circle is the ground and the airfield with the tower and the runway. The cylinder on top of this green circle represents the area served by the tower: the Control Zone (CTR). The sight of eye from the tower.
The dimensions of the areas (the cylinders) are mentioned in the relevant publications and often on the navigation charts.
Hereunder left, example from the AIP of the Netherlands of the airfield of Groningen (Eelde) with its TMA (dimensions lateral and vertical from 1500 ft AMSL until FL065) and in the middle the dotted line with the Control Zone, the TWR area (dimensions lateral and vertical from the ground till 3000 ft AMSL).
N.B. Amsterdam FIR means Flight Information Region. That is the whole of the Netherlands airspace (see further).
Now let’s take two airfields and we will put one big cylinder (area) around them. That’s where we put the ‘area control’.
The control in between the airfields and the bigger control area (CTA) around them.
Finally, to indicate an even bigger totality, we have added several areas, all connected and all with an approach control and one or more airfields.
Area control 1 | Area control 2 | Area control 3 | |||
Approach 1 | Approach 2 | Approach 3 | Approach 4 | Approach 5 | Approach 6 |
Airfield 1 | Airfield 2 | Airfield 3 | Airfield 4 | Airfield 5 | Airfield 6 |
Let’s have a look what it means for ATC, if there is a flight from airfield 1 (left) towards airfield 6 (right).
From left to right the pilot is subsequently under control of:
DEL 1 | GND 1 | TWR 1 | APP 1 | CTR 1 | CTR 2 | CTR 3 | APP 6 | TWR 6 | GND 6 |
flightplan delivery | taxi | Take-off | departure | climb + overflight | overflight | overflight + descent | approach | landing | taxi |
All these different services work very closely together. They are link like a chain. That means that they are communicating with each other as well. They co-ordinate with each other and they have standard procedures on the transfer of aircraft from the one controller to the next. It has to be one fluent stream of traffic.
THE CHAIN
ATC is like a chain with links. And the strength of this chain is determined by the weakest link.
That’s one comparison.
There is another one. The stream of traffic is like a funnel. On top you can only throw-in such an amount of liquid as can stream out on the bottom of the funnel. If you throw in too much, you’re shoes may get wet.
The combination of these comparisons shows that we have to be careful with overloading the weakest link of the chain. Otherwise the whole system may collapse.
In real-life it is know that if there is heavy fog at an important European airfield like London, you may see the effects throughout the rest of the European ATC system. Why?
Aircraft move from one air traffic control unit to the other. There is a constant stream of traffic, from departure until landing. And this stream does not allow for interruptions.
In our example here above a flight is on its way from airfield 1 to airfield 6, this aircraft is controlled by the different ATC units. If there is an interruption in one of these services, there will be immediately a traffic jam. If the one unit cannot hand-over its traffic to the next unit, the funnel becomes overloaded and …there will be holdings, effecting up-stream the traffic flow in the previous units.
A traffic jam in the air is not always easy to handle. Therefore, keep the aircraft on the ground until the problems in the air are solved. Don’t let them take-off for a while.
Still, in reality there are several places where aircraft can hold in the air. The so-called holding-patterns. Normally around airfields, but since the capacity of a holding-pattern is limited as well, there may be en-route holding patterns.
Simply said: If the funnel at the airfield is getting full, than en-route has to stop filling the funnel.
That is the theory.
In real-life it is a bit more complicated. But one thing is the same: There has to be a perfect coordination between all the air traffic units involved. The chain is as strong as its weakest link.
Every controller is responsible for his own part of the airspace or airfield. Still he is part of that chain. This means to the up-stream control he has indicated, what he wants and is able to do. And that he has to ask to the down-stream control if he is allowed. Always there are two sides involved.
If an Approach controller is starting to drown, he will refuse traffic from Area control. Or he will keep departures waiting.
Always try as a controller to avoid an overflow (of the funnel) by timely giving signals into the chain.
A closer look to the flight-plan
Note: There is more information on the flight-plan in this blog at ATC, the chain
Let’s go a few steps backward.
The pilot makes a flightplan. In there it is exactly indicated, what he has in mind. The callsign, type of aircraft and its speed, airfield of departure and destination, flightplan routing with the airways in between them and the SID (Standard Instrument Departure) point.
A flight-plan is a planning tool that indicates everything that the pilot may know and what he wants to do. But there are three points still missing when the pilot makes his plan:
- the actual squawk code
- the (real) departure time
- the cleared flight-level
These three points are still unknown at the moment the flightplan is being filed.
At the moment when these three points become known, they can be put into the flightplan. With that, the flightplan is becoming “active”. One could say, that it is the moment the flightplan will start working. It is like a pre-planned scenario. As from the moment the flightplan becomes “active”, all events thereafter could be calculated. And thus made known.
An example: The flightplan of a Amsterdam departure indicates: EHAM LEKKO B31 WOODY NIK EBBR. This means that the aircraft will follow the standard and cleared departure route for the runway involved towards the point LEKKO (the SID point). Thereafter it will follow airway B31 via WOODY and NIK. Next will be the clearance for a STAR for the active landing runway in Brussels.
Now suppose take-off runway in Amsterdam is RWY 24, the flying time via the standard departure route towards LEKKO is 5 minutes, the flying time from LEKKO to WOODY is 6 minutes and from WOODY to NIK is 3 minutes.
When we know at what time this aircraft will take off, or has taken off, then we know as well at what time it will enter the Belgium airspace at WOODY. Calculation: T/O time + 5 + 6. That’s 11 minutes after take-off with an average speed of e.g an B737, this aircraft will be under control of a Belgium ATC unit.
Why this, maybe, complicated whole story? To be able to control air traffic.
Because these calculations can be done with any active flightplan. That means that for every active flight it may be known at what moment what event will take place in time. Or, at what moment an aircraft will arrive where and at which altitude.
Remember the story here above about the funnel? With the knowledge of the information of all active flight plans we could make calculations about where there may be an overload in the system. Where holdings may be necessary. Where delays can be expected.
In real life, this is being done through so called tactical flow management. Based on actual calculations, traffic predictions could be made.
In addition there is long term flow management to provide landing and take-off licences for airfields. One of the reasons here why holiday flight are always very early in the morning, it is their take-off licence. They have to be away before the regular airlines start their flights for the business-men. In other words, spreading the traffic to avoid an overload of the funnel.
The data presentation
With the help of radar we are able to see the air traffic and to project that on a screen. We look at the radar screen and we make an image of the traffic in our minds. Literally, because the radar screen is a flat screen and the imagine has to be spatial, 3 dimensional. In the air space there is one dimension more than on the flat radar plane. The exact position of an aircraft in the air is determined by a direction and a distance from a known point and in addition by the altitude above the ground.
Example: On the radar picture here right we see on the left KLC056. On the right at EHAM there is the indication of the direction to and the distance from the aircraft to EHAM (090/060nm). To make the picture fully spatial there is one item missing: The altitude. And that we can see in the aircraft label on the radar, F176.
These three elements, direction, distance and altitude, together determine the exact position of the aircraft in the airspace.
It means that the air traffic controller has to add in his mind this altitude dimension, when he looks at the radar picture.
A modern radar gives a lot of information and depicts all that on the screen. This is called the digital radar picture. But how does it work?
In fact simply by coupling the radar information and the flight-plan information. And hereby the SSR squawk code is the actual coupling factor. A short explanation.
The radar sees the aircraft in the air and shows this as a symbol on the radar screen.
The flightplan gives all the information about that aircraft and calculates the events, as from the moment the flightplan has been activated.
With the squawk code we tell the system which flightplan has to be coupled to what symbol. The so-called "correlation".
In other words: By coupling the flightplan information with the radar information we make the digital radar picture.
With that picture the air traffic controller can make an image of the situation in the air and with that image in mind he provides proper separation of the aircraft under his control. He instructs aircraft to fly a specific heading and/or he allocates different altitudes.
That is in short the task and the work of an air traffic controller!
CONCLUSION:
As you may have seen the air traffic controller is a busy guy. He keeps an eye on the traffic, he applies different separations and he makes co-ordinations. He applies different traffic rules and he relieves the pilots of some work. He accompanies their flights and he supports them. He provides a service.
Without pilots an air traffic controller cannot do his job. The other way around the work of a pilot is easier and above all more safe with the help of air traffic control.
In other words: It is a working together. Based on rules and agreements. It is a team.
Have a save flight! With ATC :)
Bob (ATCO) van der Flier
© bobatco Utrecht
© All rights reserved by Studio Esmeralda doc ATC V2.1 eng - -May 2010
Published earlier and now edited for renewed publication
This 'full' story as a pdf-file available for your convenience, with the proper respect to copy-rights
Enjoy :)
ATC is a service ..........
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